The present invention relates generally to the field of scanning electron microscopes and more particularly to scanning electron microscopes used as test probes for visualizing and testing integrated circuits.
As a result of progress in the design and fabrication of integrated circuits, it has become possible to create circuits having millions of conductors and transistors in which the individual conductors and nodes are of the order of one to two microns. These circuits are too small and complex to be amenable to testing and analysis by techniques using mechanical probes. The mechanical probes tend to capacitively load the circuits under test thus altering the behavior one wishes to measure. Further, the mechanical probes may actually physically damage the minute conductors and nodes with which they come in contact. Finally, the number of nodes which must be examined to debug a VLSI integrated circuit is rapidly becoming too large to be amenable to manual measurement, one node at a time. As a result, test probes based on electron beams have been developed. These test probes provide a means for measuring the potential on minute conductors as well as a means for forming an image of the conductors and the surrounding circuitry without any physical damage thereto.
FIG. 1 illustrates the basic principle on which electron beam test probes operate. When a beam of electrons of sufficient energy strikes a conductor, secondary electrons are emitted having an energy distribution which is a function of the potential on the conductor. Typical energy distributions for conductors at ground and -5 V are shown in FIG. 2. In addition to the secondary electrons, some of the electrons from the electron beam are also scattered in the backward direction by the conductor which gives rise to the small peak at or near the energy of the electron beam. These electrons are referred to as backscattered electrons. If the conductor under bombardment is at a negative potential of 5 volts, then each of the secondary electrons leaving the conductor will have an additional energy of 5 volts resulting from the repulsion of the electron by the negative potential of the conductor. Hence, the energy distribution shown in FIG. 2 for a conductor at ground potential will be shifted by 5 volts when the conductor is at a negative potential of 5 volts. Conversely, if one observes the energy distribution of secondary electrons emitted from the conductor under bombardment, one can in principle deduce the potential of the conductor.
Prior art electron beam test probes have been based on scanning electron microscopes. These are best exemplified by the work of Plows, e.g., U.S. Pat. No. 3,628,012. The apparatus taught by Plows is shown schematically in FIG. 3. An electron beam from a scanning electron microscope was directed on the conductor under observation. The secondary electrons leaving the conductor with sufficient velocity in a direction parallel to the electron beam to overcome a retarding field generated by a filter grid were measured by a detector.
The Plows apparatus has several problems. First, the measured value for the potential of the conductor under bombardment is dependent on the electric fields in the neighborhood of the point of bombardment. This is primarily the result of the means chosen to measure the energy of the electrons, i.e., choosing to measure the component of the secondary electron velocity parallel to the direction of the incident electron beam. Referring to FIG. 4, each secondary electron may be characterized by a velocity v and an angle of emission, T, relative to the direction of the electron beam. The number of secondary electrons emitted at an angle, T, is proportional to the square of the cosine of T. The secondary electron velocity vector v can be decomposed into a component, v.sub.p, parallel to the direction of the electron beam and a component, v.sub.r, perpendicular to the direction of the electron beam. Since the retarding field generated by the filter grid only affects the v.sub.p component of the velocity, the number of electrons which succeed in traversing the grid is a measure of the number of secondary electrons with v.sub.p greater than some predetermined value, not a measure of the number of electrons with total velocity greater than a predetermined value. As a result, secondary electrons emitted with large velocity and T at or near 90 degrees will not be counted, and secondary electrons emitted with much smaller velocities at or near 0 degrees will be counted.
Another problem with prior art electron beam test probes is that the angle of emission, T, may be altered by electric fields in the vicinity of the point of emission of the secondary electrons, as illustrated in FIG. 5. Here, a secondary electron which is emitted at an angle near 90 degrees, passes over a second conductor which is at a negative potential. This negative potential gives rise to a local electrical field which deflects the secondary electron in question, resulting in its effective angle of emission being decreased. As a result, a secondary electron which would not have been counted by the detector will now be counted. This results in "cross-talk" between the conductor being probed and the neighboring conductor. A signal on the neighboring conductor will appear as a signal on the conductor being probed, since the potentials generated by the signal on the neighboring conductor will increase or decrease the number of electrons counted by the detector whenever said signal becomes negative or positive, respectively. This is a serious problem in the prior art designs.
In addition to cross-talk, the sensitivity of the measured potential to local electric fields results in drift in the measured potential as the result of slow changes in the surface potential of the circuit being probed. The regions between the conductors on the integrated circuit accumulate charge as a result of bombardment by the electron beam when it is moved from one point to another. Such charge build-up also occurs as a result of scanning the entire circuit area to form an image of the circuit prior to deciding which point on the circuit is to be probed. Since these areas of the circuit have relatively high resistivity, this charge results in electric fields which slowly decay in time. Since the prior art systems are sensitive to electric fields in the neighborhood of the point being probed, these systems display long term drift in the measured potential as a result of the slow discharge of the areas between the conductors.
The prior art solution to this problem was to place an extraction grid at a large positive potential just above the surface of the circuit being tested. This grid attracts the secondary electrons away from the surface of the circuit so as to minimize the effects of surface electric fields. Unfortunately, this grid is not capable of overcoming all of the surface potential effects. In particular, it does not eliminate cross-talk between neighboring conductors. In addition, this grid often results in the surface of the circuit becoming highly positively charged, since any electrons which escape the surface of the specimen are swept away by this grid. Hence, if the electron beam is of an energy at which more than one secondary electron is emitted by the specimen for each electron absorbed, the specimen will become increasingly positively charged until the surface charge reaches a value which neutralizes the extraction grid field. This surface charge may also create adverse effects on the operation of the circuit under test.
Stray electric fields in other parts of the apparatus also affect the measured potential by either altering the trajectories of the secondary electrons or by altering the trajectory of the electron beam. For example, the extraction and filter grids must be positioned and supported by insulators. These insulators collect electric charges in a non-reproducible manner as a result of being struck by electrons which are not completely collimated into the electron beam, by backscattered electrons, and by secondary electrons. The prior art solved this charge buildup problem by employing complicated grounded shields the insulators in question which prevented stray electrons from striking the insulators and which shielded the electron beam and the secondary electrons from any fields which were produced by charges that made it past these shields and accumulated on the insulators in question.
Similarly, stray electric fields can affect the electron beam deflection system. The electron beam is often swept across the circuit by magnetic deflection. The electron beam must pass through the sweep magnets in a tube which must be conductive enough to prevent charge build up but resistive enough to prevent eddy currents from interfering with the response time of the sweep magnets. One prior art solution to this problem employs an insulating tube with a thin layer of carbon on the inside surface. This solution is economically unattractive.
The considerable space needed between the specimen and the last magnetic lens in the electron beam optical system to accommodate the filter and extraction grid assemblies in prior art systems gives rise to a second major deficiency in these systems. This space is determined by two considerations. First, the electron beam and secondary electrons must pass through these grids; hence the space between the wires used to construct the grids must be large compared to the diameter of the wires. If it is not, a significant fraction of the electron beam and secondary electrons will be intercepted by the grids. Second, in order for such an open grid to be an ideal potential barrier, the distance between the extraction grid and the filter grid must be large compared to the spacing between the wires that make up the grids. As a result of these two considerations, a significant distance, D, must exist between the bottom of the last focusing magnet in the electron beam column and the specimen as shown in FIG. 6. This large space limits the spot size of the electron beam on the conductor being probed.
Referring to FIG. 6, the last magnet in the electron beam column focuses the electron beam to a spot on the specimen at the point being probed. The minimum size of this spot is determined by the chromatic aberration of this magnetic lens. The chromatic aberration of the magnetic lens is roughly proportional to its focal length. The focal length of the prior art magnetic lens used for this purpose was at least as long as the distance from the bottom of the lens to the specimen. Hence the space needed to accommodate the extraction and filter grids forced the prior art systems to use a long focal length magnetic lens which in turn resulted in a large chromatic aberration. This chromatic aberration limits the spatial resolution of the prior art systems.
The fact that prior art systems were essentially modified scanning electron microscopes, results in a third class of problems. A normal scanning electron microscope is optimized to have the best possible spatial resolution. A resolution of 5/1000 of a micron is not unusual. This requires a high energy electron beam, typically 10 to 20 keV. The problems created include first, that this high energy beam can interfere with the operation of a circuit being tested. Second, it produces high backgrounds of electrons from backscattered electrons striking the walls of the containment vessel. These electrons limit the signal-to-noise ratio of the probe. Finally, it prevents the construction of a small compact instrument.
More specifically, a 20 keV electron beam can penetrate a significant distance into the surface of the integrated circuit being tested. This penetration can result in the charging or discharging of isolated gate elements in the circuit being tested. Such charging and discharging can result in permanent circuit damage.
Second, a significant fraction of the electron beam is backscattered by the specimen being tested. These backscattered electrons strike the walls of the vacuum vessel and produce secondary electrons with energies in the range detected by the electron detector. The number of such secondary electrons produced by each backscattered electron increases as the energy of the backscattered electrons increase. At 20 keV, these secondary electrons are a significant source of noise in the test probe instrument.
Finally, the physical size of the apparatus is constrained by the energy of the electron beam, the larger the electron beam energy, the larger the physical size of the apparatus. In most scanning electron microscopes, the electron beam is caused to scan the specimen being tested by magnetic deflection coils. The size and power dissipation of the deflection coils and the magnetic lenses used to focus the electron beam are directly related to the energy of the electron beam. At 20 keV, these coils dissipate sufficient power that they must be cooled, which complicates the physical design. To accommodate this cooling, the magnetic deflection coil is usually located outside the vacuum chamber. This requires that the vacuum chamber contain a tube which passes through each magnetic deflection coil. This geometry complicates the vacuum vessel and increases its cost. In addition, the distance from the magnetic lens to the specimen increases with increasing electron beam energy. This constrains the minimum height of the electron beam column.
Prior art test probe systems have also not been designed so as to be useable by someone who has not been trained in the maintenance and use of electron microscopes. They generally have been complex machines which require a great amount of tuning by the operator before desired images can be obtained.
Scanning electron microscopes are optimized for making images of small areas on the specimen. Test probe systems must be optimized for measuring rapidly changing voltages at a specified point which may be anywhere in the integrated circuit.
This leads to two further problems when such devices are adapted for use as test probes. First, the physical size of a typical VLSI integrated circuit is much larger than the "field of view" of the typical scanning electron microscope. As a result, only a small portion of the circuit may be probed at any one time. Second, the time required to measure the potential at a specified point is often too long. Since the measured potential is the average of the potential on the conductor over the time required to make the measurement, this can lead to inaccuracies if the potential on the conductor changes significantly over the potential measurement.
To avoid this second problem, the electron beam must be pulsed in very short pulses at precisely defined times relative to the test signals which govern the operation of the circuit. This imposes design constraints which are not properly incorporated in the design of the prior art systems. For example, the timing circuits used to control the electron beam pulsing often limit the frequency of the electrical signals that can be measured by these systems.
Finally, integrated circuits often contain conductors which are covered by a layer of insulating material. A means for measuring the potentials on such buried conductors is needed. The potentials on these conductors give rise to electrostatic potentials on the surface of the insulating layer. Thus it should be possible to measure the potential of the underlying conductor. However, prior art systems have not provided a satisfactory method for measuring these potentials. In prior art systems, the electron beam bombardment of the insulating material surface led to a change in potential of said surface which in turn made it difficult to deduce the potential of the underlying conductor.
Consequently it is an object of the present invention to provide an electron beam test probe system which is less sensitive than prior art systems to electric fields at points on the specimen other than at the point on the circuit which is being probed.
It is another object of the present invention to provide an electron beam test probe system which has an improved signal-to-noise ratio.
It is a further object of the present invention to provide a compact, easy to use electron beam test probe system which is suitable to production line use.
It is yet another object of the present invention to provide a test probe that can measure the potential on a VLSI integrated circuit at any point within the physical boundaries of said integrated circuit even though said boundaries may exceed the field of view of a scanning electron microscope.
It is a still further object of the present invention to provide an electron beam test probe system which can measure high frequency electrical signals.
It is a still further object of the present invention to measure the potentials of conductors covered by an insulating layer.
Finally, it is an object of the present invention to provide a test probe system which can produce an image of the circuit being tested while simultaneously measuring the potential at one or more of a plurality of selected points in the circuit being tested.
These and other objects of the present invention will be apparent from the following detailed description of the present invention and from the accompanying drawings.